docs: def _encode_time(self, value):
          """Convert datetime to base64 or plaintext string"""
  
          if self._kp.version >= (4, 0):
              diff_seconds = int(
                  (
                      self._datetime_to_utc(value) -
                      datetime(
                          year=1,
                          month=1,
                          day=1,
                          tzinfo=tz.gettz('UTC')
                      )
                  ).total_seconds()
              )
              return base64.b64encode(
                  struct.pack('<Q', diff_seconds)
              ).decode('utf-8')
          else:
              return self._datetime_to_utc(value).isoformat()